The invention relates to a display apparatus having at least one electronically controllable display element. More specifically, the invention relates to a display apparatus that makes use of the electrowetting effect.
It is known to implement display apparatuses with the aid of colored liquids, for which purpose the liquids can be shifted between positions visible and not visible to an observer. An efficient means for shifting the liquids is so-called electrowetting, in which an electrode arrangement present in the immediate vicinity of the liquid droplet is impinged upon by a voltage, with the result that an electric field acts on the liquid droplet, the consequence of which is that the surface energy and thus the surface tension of the liquid rises. Depending on the arrangement, this can cause the liquid to spread out on the electrode or in fact to cover it completely, depending on the geometry and/or surface condition exhibited by the electrode, and on the relative positions also assumed by the liquid and the electrode with respect to the earth's gravitational field. It is known to equip the electrodes with a hydrophobic coating in order thereby to achieve a maximum effect between the state of the liquid with the electrode impinged upon and not impinged upon. With the aforesaid means it is already possible to implement droplet motion with no need to resort to further functional elements.
The local surface energy of the liquid can thus be modified, and the geometry of a droplet thereby locally changed, by means of a local variation of the electric field strength. In particular, the generation of a locally elevated surface energy can cause a movement of the droplet, by the fact that the latter expands preferentially into the corresponding region characterized by the presence of an electric field. Conversely, the surface voltage causes liquid to be is pulled back, by the influenced part of the droplet, out of the region in which it is uninfluenced and thus exhibits a lower surface energy, since in this region it “seeks” the lowest-energy geometry, namely that of a sphere. The result is a net transport of liquid out of the region of lower field strength into the region of higher field strength.
A variety of displays have already been implemented on this basis. The goal in this context was usually always to produce an electronically controllable bistable display element that selectably assumes one or the other state. It is thus known, for example, to transport a liquid with the aid of the electrowetting effect back and forth between a first and a second volume that are connected to one another via a conduit, for which purpose each volume comprises an assigned electrode so that the transport direction results in each case from the field-strength relationship between the two electrodes. By preference, only those respective electrodes in whose direction the liquid is intended to be transported are impinged upon by a potential.
WO 2009/036272 A1 discloses an electrowetting-based display apparatus in which each display pixel has associated with it an electrode pair that constitutes a capacitor, with the aid of which a liquid that is present in a visible volume of the pixel can be impinged upon by an electric field. Return transport of the polar or electrically conductive liquid into a reservoir, which is located below the visible display surface in the viewing direction toward the display element, is achieved by the fact that the geometries of the visible display volume and those of the reservoir are coordinated with one another in such a way that the polar or electrically conductive liquid, when it is not impinged upon by an electric field, is drawn into the reservoir by Young-Laplace pressure.
If it were desired to implement a multi-color display with the aid of the technology recited above, this would be possible only with the aid of a plurality of pixels, arranged next to one another on the display surface, that comprise differently colored liquids. It is not possible, however, to operate the apparatus according to WO 2009/036272 A1 in such a way that a single pixel can assume more than two color states.
EP 0 806 753 describes a display apparatus having at least one electronically controllable display element, in which apparatus each display element has associated with it a separate electrode pair for controlling a liquid held in a holding volume. An embodiment is described in which the display element encompasses a visible sub-volume that is fluidically connected to three holding volumes.